The present invention relates in general to sensors and, more particularly, to interface circuits for sensors.
Microelectronics technology is presently being used to fabricate mechanical and electromechanical sensors that convert a physical condition to an electrical sense signal. Such sensors are often integrated on a semiconductor substrate with circuit components. For example, where the physical condition is an acceleration, sensing is typically performed by an accelerometer manufactured using micromachining techniques and processing steps similar to those used for fabricating integrated circuits. Accelerometers are used in automobile applications such as collision detectors in air bag systems and as vibration sensors in appliances such as washing machines.
In many applications, the sense signal produced by an accelerometer is coupled to an analog-to-digital converter (ADC) which converts the sense signal to digital data for further processing. Some ADCs have ratiometric inputs, so that the accuracy of the analog-to-digital conversion depends on the sense signal being ratiometric as well. A ratiometric signal is one that is proportional to the power supply voltage. For example, if a sense signal V=0.5*V.sub.DD, where V.sub.DD is a power supply voltage, so that V=2.5 volts when V.sub.DD =5.0 volts and V=2.0 volts when V.sub.DD =4.0 volts, then V is ratiometric.
Many prior art accelerometers produce non-ratiometric sense signals. A non-ratiometric accelerometer produces a sense signal having a particular signal level in response to an acceleration independent of power supply voltage. For example, a sensor signal may have a voltage level of V=2.5 volts when V.sub.DD =5.0 volts or when V.sub.DD =4.0 volts. Such sensor signals are non-ratiometric. Over a range of power supply voltages, ratiometric signals typically are not equal to non-ratiometric signals.
In converting an incoming sense signal to digital data, an ADC that has a ratiometric input interprets the incoming sense signal as being ratiometric, i.e., as having a ratiometric amplitude, even when the incoming sense signal is non-ratiometric. If the sense signal is non-ratiometric, an error in the digital data which corresponds to the difference between the ratiometric and non-ratiometric amplitudes is generated by the ADC. An error in the sensor signal can have dramatic undesirable consequences especially in safety driven applications such as air bag systems.
Other prior art accelerometers produce a ratiometric sense signal by multiplying a ratiometric reference signal by a non-ratiometric sense signal. The multipliers tend to be overly complex and consume excessive die area.
Hence, there is a need for a ratiometric sensor that does not use multipliers and therefore consumes less die area.